1. Field of the Invention
The invention relates to a cellular digital radio transmission system employing time-division multiplex message channels between base stations and mobile stations in such system.
2. Description of the Prior Art
There are three known basic methods for sending digital messages over a transmission medium (e.g. transmission line, radio channel): these are code-division multiplexing, frequency-division multiplexing and time-division multiplexing.
In the code-division multiplexing method the different messages sent over a common transmission medium are imposed, for example, by baseband modulation, on a carrier and the resultant narrow-band signal (narrow in relation to the bandwidth of the channel) is spectrally spread over the channel bandwidth by multiplex modulation using a codeword that characterizes the receiver.
Recognition of the signal at the receiver takes place not by time-division or frequency-division selection but on the basis of the spectral coding. The multiply stacked spectrally-coded messages in the code-division multiplex channel are selected at the receiver on the basis of their allocated codewords.
In the frequency-division multiplexing method the whole bandwidth available for message transmission is divided into narrow frequency bands, each of which corresponds to one message transmission channel. This narrow frequency band is at the disposal of the user for the duration of the message transmission.
In the time-division multiplexing method each user has available to him the whole bandwidth of a single transmission channel but only for short intervals of time. The characters or sequences of characters of various users are interleaved and are sent out with a correspondingly higher bit rate, the time channel, allocated to each user being periodically repeated after the duration of each frame period.
From West German Patent DE-OS No. 25 37 683 a radio transmission system with base stations and mobile stations is known, in which different channel access methods are used with asynchronous time-division multiplex, code-division multiplex and frequency-division multiplex.
Also known are combinations of the aforesaid methods and their application in a digital radio transmission system. For example, in "Nachrichtentechnik, Elektronik+Telematic 38 (1984), Vol. 7, pages 264 to 268" a digital radio transmission system is described in which the time-division multiplex method is used in combination with spectral coding, but in which the various users are not separated by means of the code-division multiplex method. In the time channels for speech and/or data message transmission (TCH communication channel) a bit sequence for establishing synchronism, a frame-synchronization word and the bit sequence of the message itself are transmitted one after the other. The time channels for message transmission (3.times.20 TCH) are arranged with organization channels (3 CCH) to form a time-division multiplex frame with a duration of 31.5 msec. If the message to be transmitted is a speech signal, adaptive delta modulation can be used for analog/digital conversion. A code is superimposed on the resultant message characters (bits) in the sender. It has proved advantageous to group the individual message bits into blocks of four bits each and to spread the resultant blocks with an orthogonal alphabet. The spreading factor used is a compromise designed to combine the advantages of band spreading with the requirement to use the frequencies economically.
As appears from the foregoing, it is common practice in radio transmission systems to transmit a multiplicity of communication channels by time-division multiplexing. The time-division multiplex frame is thereby divided into a fixed time slot raster and each mobile station is informed by the base station of the time slot in which the mobile station can receive or transmit information. Such a time-division multiplex method is advantageous when all users in the transmission system require the same channel bandwidth for the transmission of messages. However, if there are users in the system with different bandwidth requirements, the channel capacity available to users with low bandwidth requirements is not fully utilized. This is particularly disadvantageous in message transmission over radio channels, since the system bandwidth in a radio transmission system is in any case limited. Such users with different bandwidth requirements are found in data transmission for various services such as for example telefax, teletext, computer communications, etc. With the advance of technology, it is likely that in future there will be less bandwidth needed for speech transmission, especially in the case of analog/digital conversion. The later introduction of such a new generation of mobile radio stations with a lower bandwidth requirement is only conditionally possible with a time-division multiplex frame divided into a fixed time slot raster.
From West German Patent DE-PS No. 31 05 199 a method is known for transmitting data packages from several auxiliary stations to a main station using a common channel, in which the allocation of the time slot to the auxiliary stations or users is performed dynamically. Dynamic allocation means that a user, after access, is allotted as many time slots as he needs for transmitting the information, whereby the number of the time slots changes from access to access and from user to user. A method of this type is suited to the transmission of messages and of data packages that are not critical in terms of time, since the time between two successful accesses cannot be determined, that is to say no fixed data transmission rate can be guaranteed. Such a method cannot be used, however, for transmitting speech, because in the transmission of a speech signal it would be necessary during a conversation to have renewed access to the common transmission channel in accordance with the method indicated in DE-PS No. 31 05 199. Whether such attempted access is successful or not depends on the volume of traffic, so that this method cannot be used for the transmission of messages and of data that are critical in terms of time and are of different magnitude, though transmitted at a constant data rate. The aforesaid West German Patent DE-PS No. 3105199 corresponds to applicant's copending U.S. application Ser. No. 841,134, filed Mar. 18, 1986. That application discloses transmitter and receiver arrangements such as those described herein, but is directed to synchronization of the base station transmitter with the mobile station receivers.
The object of the invention is to enable optimum utilization of channel capacity in a cellular digital radio transmission system in which messages are grouped together by the time-division multiplex method for transmission over a common channel.
This object is achieved in accordance with the invention by providing time slots of different durations in each TDM time frame, thereby permitting the mobile stations to be assigned to time slots corresponding to their bandwidth requirements.
A digital radio transmission system in which the time slots in the time-division multiplex frame have different durations has the advantage that the channel capacity is optimally utilized by users with different bandwidth requirements. Since the useful channel capacity of the time-division multiplex frames can be divided in arbitrary steps depending on the requirements of the users, adaptation will be possible to future system extensions.